Dependencies of Shear Wave Velocity and Shear Modulus of Soil on Saturation

Shear wave propagation in soil is a physical phenomenon and has been used widely for monitoring and seismic property assessment in geotechnical engineering. Shear wave velocity $V_s$ and small-strain shear modulus $G_0$ are the key parameters in defining material response to various dynamic loadings. To date, the dependencies of $V_s$ and $G_0$ on saturation, especially in high suction range, are still not well understood because of the limited testing methodology and experimental evidence. In this study, the authors present a new laboratory instrumentation of measuring shear wave propagation in different types of unsaturated soils. Low relative humidity and water mist injection environment are used for measuring shear wave velocity under both drying and wetting conditions. Bender element technique was used to measure the shear wave responses. Step function was used as excitation, and determination of a first arrival time was identified and consistently used for all shear wave measurements. Shear wave evolution and the calculated $V_s$ and $G_0$ with varying volumetric water content under zero total stress condition along drying and wetting are presented. The effects of different soil–water regimes on the evolution of $G_0$ are examined. It is found that $V_s$ or $G_0$ depends highly on soil types, saturation, and drying/wetting state. Parameter $V_s$ or $G_0$ is the lowest when a soil is saturated and the highest when it is dry—varying from tens of meters per second for $V_s$ or a few megapascals for $G_0$ at full saturation for all soils to up to $\mathrm{1,800}\mathrm{m}/\mathrm{s}$ of $V_s$ or 2 GPa of $G_0$ in clayey soil at dry state. The variability of $V_s$ or $G_0$ on soil type becomes more pronounced as soil has more clayey materials. It is also identified that hydraulic hysteresis of $V_s$ or $G_0$ is prominent only in the capillary water retention regime for all types of soil.